1. Field of the Invention
The invention relates generally to drill bits which have polycrystalline diamond compact (“PDC”) cutters thereon.
2. Background Art
Polycrystalline diamond compact (“PDC”) cutters have been used in industrial applications including rock drilling and metal machining for many years. In a typical application, a compact of polycrystalline diamond (or other superhard material) is bonded to a substrate material, which is typically a sintered metal-carbide to form a cutting structure. A PDC comprises a polycrystalline mass of diamonds (typically synthetic) that are bonded together to form an integral, tough, high-strength mass or lattice.
An example of a rock bit for earth formation drilling using PDC cutters is disclosed in U.S. Pat. No. 5,186,268. FIGS. 1 and 2 from that patent show a rotary drill having a bit body 10. The lower face of the bit body 10 is formed with a plurality of blades 16-25, which extend generally outwardly away from a central longitudinal axis of rotation 15 of the drill bit. A plurality of PDC cutters 26 are disposed side by side along the length of each blade. The number of PDC cutters 26 carried by each blade may vary. The PDC cutters 26 are individually brazed to a stud-like carrier (or substrate), which may be formed from tungsten carbide, and are received and secured within sockets in the respective blade.
A PDC cutter may be formed by placing a cemented carbide substrate into the container of a press. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate and treateed under high pressure, high temperature conditions. In doing so, metal binder (often cobalt) migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn bonded to the substrate. The substrate often comprises a metal-carbide composite material, such as tungsten carbide. The deposited diamond layer is often referred to as the “diamond table” or “abrasive layer.”
One of the major factors in determining the longevity of PDC cutters is the strength of the bond between the polycrystalline diamond layer and the sintered metal carbide substrate. For example, analyses of the failure mode for drill bits used for earth formation drilling show that in approximately one-third of the cases, bit failure or wear is caused by delamination of the diamond table from the metal carbide surface.
Many prior art PDC cutters have the diamond table deposited on a substrate having a planar interface. However, in an attempt to reduce the incidents of delamination at the PDC/metal carbide interface, several prior art systems have incorporated substrates having a non-planar geometry to form a non-planar interface. U.S. Pat. No. 5,494,477 discloses cutters having a non-planar interface. FIG. 3 illustrates one embodiment of a PDC cutter having a non-planar interface. As shown in FIG. 3, PDC 110 includes a plurality of sloped surfaces 114, 115 between the substrate 111 and the abrasive layer 112.
Additionally, other prior art systems have incorporated an intermediate layer between the diamond layer and the substrate to reduce these stresses. U.S. Pat. No. 5,510,193 discloses an intermediate layer of polycrystalline cubic boron nitride between a PDC layer and a cemented metal carbide support layer. Further, in the '193 patent, the metal binder, i.e., cobalt, is substantially swept from the metal carbide support layer into the intermediate layer and into the PDC layer. The '193 patent contributes the observed physical properties and interlayer bond strengths of the '193 compact to the sweeping through of the cobalt into the intermediate and PDC layers.
Furthermore, an additional factor in determining the longevity of PDC cutters is the heat that is produced at the cutter contact point, specifically at the exposed part of the PDC layer. The thermal operating range of PDC cutters is typically 750° C. or less. Temperatures higher than 750° C. produce rapid wear of the cutter because of differential thermal expansion between cobalt and diamond in the PDC layer, which may result in delamination. This thermal expansion also jeopardizes the bond strength between the diamond table and the carbide substrate.
Accordingly, there exists a need for thermally stable PDC cutters having a decreased risk of delamination.